1. Field of the Invention
The present invention relates to an optical information recording medium which can be recorded and/or reproduced by a magneto-optical recording apparatus.
2. Description of the Prior Art
Optical data storage media such as a compact disc (CD), a laser disc (LD) or the like are now commercially available on the market. Each of these optical data storage media comprises a substrate on which concavities and convexities corresponding to information signals are previously formed, a reflecting layer formed on the substrate and a protecting layer formed on the reflecting layer, if necessary. This type of optical data storage medium is a playback type optical disc from which an information signal previously recorded can be reproduced only. The above reflecting layer is made of a material such as Al, Au or the like having a high reflectivity, i.e., low absorptance.
Recently, as an optical data storage medium in which information can be written and erased unlike the optical information media such as the CD or the like, the development of a magneto-optical disc has been promoted. This magneto-optical disc comprises a substrate and a recording portion formed on the substrate by laminating a recording magnetic layer (e.g., a rare earth-transition metal alloy amorphous thin film such as TbFeCo or the like) having a magnetization easy axis vertical to the film surface and which has a large magneto-optical effect, a reflecting layer and a dielectric layer. As such magneto-optical disc, a magneto-optical disc (so-called mini-disc) having a diameter of 64 mm has already been manufactured as popular magneto-optical discs.
If the above-mentioned playback optical disc can be reproduced by the magneto-optical disc recording and reproducing apparatus, then the playback optical disc and the magneto-optical disc system will become more widespread and useful in actual practice. The playback optical disc whose reflecting layer is made of a material Al, Au or the like, however, has a reflectivity of 70% or more relative to a laser light, which reflectivity is very high as compared with the magneto-optical disc whose reflectivity lies in a range of from 15 to 25%. Also, since the playback optical disc has a different signal modulation degree from that of the magneto-optical disc, the playback optical disc cannot be reproduced by the recording and reproducing apparatus which can record and reproduce the magneto-optical disc. If two kinds of amplifiers are mounted within the above recording and reproducing apparatus, then a signal can, of course, be reproduced from the playback optical disc. In this case, the circuit arrangement of the recording and reproducing apparatus becomes complicated, which is not to be practical.